Through the years, many systems have been devised to provide a musical note level or MIDI language recording of a player's performance of a piano or other keyboard musical instrument. These devices have been used to provide real-time accompaniment or sound reinforcement for the performer. Some of these devices are complex or bulky, and require an intricate, invasive installation, for example those mounted external to the piano case, such as U.S. Pat. No. 4,768,412 and Pub 2002/0003708. There are many known ways of detecting a displacement and a displacement speed, or other note expression data of a keyboard for a musical instrument. The earliest known methods were mechanical switch structures. However, these had significant disadvantages, such as uniformity of response. Additionally, these types of switches affected key touch while playing, required time-consuming mounting, needed very tight vertical and horizontal adjustment, and generally had a high cost. For a further discussion of the problems of electro-mechanical switches see U.S. Pat. No. 4,628,786 issued to Buchla. The apparatus, according to Buchla, overcomes some of the problems and disadvantages of these prior art mechanical switch structures by providing a pickup sensor for each key that is moved within an electric field formed between a pair of stationary electrodes. Although still a mechanical system, since the pickup never contacts either of the electrodes, Buchla avoids many of the problems inherent in previous mechanical contact switches. A major advantage of the Buchla apparatus is that the voltage impressed on the electric field pickup sensor varies as a function of the position of the pickup within the electric field (i.e. voltage varies according to amount of key depression. The Buchla invention is thus a continuous position sensor providing the capability to capture greater note expression data, and provide for better velocity resolution. However, this apparatus is complex, costly, and requires time consuming installation and calibration. Additionally, the circuitry required to impress the AC voltages of equal amplitude but opposite phase on the two electrodes and the detection circuitry are complex and costly.
In response to the above inherent limitations of mechanical switch structures, various attempts have utilized types of opto-electronic switches. One of the first was U.S. Pat. No. 4,351,221 issued to Starnes, et al. This system however requires two optical LED sensors per key and utilizes the sensors in a manner that creates a double contact system, thus eliminating the advantages of continuous linear position sensor. Additionally, this apparatus requires elaborate and delicate installation of photo sensors by permanent attachment to the underside of the piano keys.
Another opto-electronic switch mechanism utilizing a stationary opto-isolator in conjunction with a light path paddle mounted on a moving key is known from U.S. Pat. No. 4,362,934 issued to McLey. However, this apparatus has the same limitations as the system issued to Starnes, et al discussed above and further, is more commonly limited to electronic keyboards.
Another optical sensing means is known from U.S. Pat. No. 4,736,662 issued to Yamamoto. In order to reduce the number of displacement speed electrical signal converting elements and provide a design for use in a limited space, this apparatus utilizes stationary optical sensor elements which are coupled to optical detector using optical fibers, and the optical sensors are positioned adjacent to moving key paddles of the key striker mechanism, where the moving paddles obstruct the optical path. Another invention is known from U.S. Pat. No. 4,768,412 issued to Sanderson. This system uses a single optical sensor per key comprising an optical transmission sensor whose light path is interrupted by a paddle connected to the keys, where an adapter plate rests on top of the keyboard to translate key motion to the sensors. U.S. Pat. App. 2004/0003708 by Buschla describes a similar system with sensors placed over the tops of the keyboard, where a reflectivity measurement is made from the surfaces of the white keys and the black keys act as photo path interrupters. Velocity measurement is made by using vertically displaced black key sensors and measuring the time interval from first to second sensor path opening as the key is depressed. The white keys and black keys photodiodes are read by an A/D converter. One drawback of this system is that the basis of measurement is completely different between white keys (declining reflectivity) and black keys (key movement time from first to second sensor).
The use of optical interrupters with paddles mounted under a keyboard to measure continuous key position is also described in U.S. Pat. Nos. 5,824,930 and 6,229,081, and 6,297,437 by Ura et al.
Another invention is known from U.S. Pat. No. 5,567,902 by Kimble et al. This device uses non-multiplexed optical encoders mounted underneath keys to determine key position, but does not address crosstalk between keys or ambient light correction, and the problem of non-linearity of reflectivity response is handled by limiting the usable range of the sensor of a 25% section that is linear with position. As all of the LEDs within an octave are simultaneously enabled, optical pollution from illumination of an adjacent key into the sensor for the key to be measured must be handled using baffles and the like. Additionally, mechanical responses from adjacent keys to the key to be measured are not considered.
Another keyboard sensor is described in U.S. Pat. No. 5,231,283 by Starkey and Williams, which utilizes an under key paddle which pivots on a hinge below the keyboard, thereby moving the paddle across an optical detector which measure the degree of path interruption. The paddle is shaped to provide a linear response with key movement, and a hinge and spring mechanism removes the requirement to attach the paddles directly to the keys, as described earlier for U.S. Pat. Nos. 4,362,934 and 4,351,221. U.S. Pat. No. 5,231,283 also describes a method to convert position and time data into accurate MIDI information.
U.S. Pat. No. 5,524,521 by Clift et al describes a sensor system for use on the hammers of a piano, where a sensor for the hammer measures surface reflectivity of the hammer, thereby extracting a velocity and duration measurement.